1. Field of the Invention
The present invention relates to a multicolor image forming apparatus which forms a color-overlapped image on a photosensitive body with toners different in color in an electrophotographic method and which transfers the image on a transfer material once. It especially relates to a copying machine, a printer or another two-color image forming apparatus which uses two-color toner to obtain a two-color image.
2. Related Background Art
In such a multicolor image forming apparatus, a surface of a rotating electrophotographic photosensitive body is uniformly charged with a charging means and thereafter exposed to a laser beam or the like in accordance with a predetermined image signal to form an electrostatic latent image thereon. Then, by applying a predetermined bias to a developing means, toner is adhered to an exposed portion of the latent image to develop an image. By repeating this plural times, an overlapped toner image of plural colors is formed, transferred in a batch once to a sheet of transfer material and fixed thereon to obtain a multicolor permanent image.
A portion of a periphery of a photosensitive body in a two-color image forming apparatus is shown in FIG. 10. The two-color image forming apparatus is provided along a rotating direction of a photosensitive body 1 with two developing means, i.e. a first developing unit 4 and a second developing unit 7. The first developing unit 4 includes a two-component developing agent constituted of a toner T1 and a magnetic carrier C. On a developing roller 41 with a magnetic roller 42 built therein, the toner T1 and the carrier C are held. The held two-component developing agent is applied by a regulating member 43 onto the developing roller 41 and conveyed to a development region in which the photosensitive body 1 is opposed to the developing roller 41. In the development region, by applying a predetermined bias to the developing roller 41, the toner is adhered to a first latent image on the photosensitive body 1 to develop and visualize a first toner image.
The second developing unit 7 includes a magnetic toner T2 which is different in color from the toner T1 in the first developing unit 4. On a developing roller 71 with a magnetic roller 72 built therein, the toner T2 is held. The held toner T2 is applied by a regulating member 73 onto the developing roller 71 and conveyed to the development region. In the development region, by applying a predetermined bias to the developing roller 71, the toner is adhered to a second latent image on the photosensitive body 1 with the first toner image formed thereon, to develop and visualize a second toner image.
In this manner, while the photosensitive body 1 rotates once, a two-color overlapped toner image is formed on the photosensitive body 1. The toner image is transferred to a transfer material once and thereafter fixed to obtain a two-color permanent image.
Usually in the aforementioned image forming apparatus, if during image formation a jam of a transfer sheet, an error of a sheet supply system or another trouble occurs, by checking its error signal, a drive motor of the photosensitive body is stopped. Also, a primary charge output, a laser exposure and an application of a bias to the developing units are cut off, thereby stopping an image forming process.
However, even when an application of a driving voltage to the drive motor of the photosensitive body is stopped, the photosensitive body 1 rotates due to an inertial force produced by its weight. Until the photosensitive body completely stops rotating, a uniformly charged region extending from a primary charging means position to a laser exposure position on the photosensitive body 1 passes the first developing unit 4. When the bias to the developing roller 41 is cut off at the same time the trouble occurs, then between the developing unit 4 and the charged region of the photosensitive body 1, an electric field is produced to attract the carrier C in the developing agent toward the photosensitive body 1. Therefore, the carrier C adheres to the photosensitive body 1.
The carrier C adhering to the photosensitive body 1 is conveyed to a vicinity of the development region of the second developing unit 7 while the photosensitive body 1 is rotating. Since the developing roller 71 of the second developing unit 7 contains the magnetic roller 72, a portion of the conveyed carrier C adheres to the developing roller 71 because of a magnetic suction force and is taken into the developing unit 7. The taken carrier C accumulates in a vicinity of an opposed portion of the developing roller 71 and the regulating member 73, thereby inhibiting application of the toner T2. Therefore, an uneven application, a resulting uneven concentration of the second toner image or another image deterioration is caused. The adhesion of the carrier to the photosensitive body and the mixture thereof into the second developing unit at the time of occurrence of jam or another trouble are detailed in the description of embodiments of the invention.
As a countermeasure to solve the problem, Japanese Patent Publication No. 7-50350 proposes a method in which to prevent the adhesion of a carrier at the time of trouble occurrence, a potential of a photosensitive body before passing a developing means is detected by a potential sensor and a bias of the developing means is controlled in accordance with a change in potential of the photosensitive body. However, in the two-color image forming apparatus, since two developing means are disposed around the photosensitive body, in some case no space can be secured for mounting the potential sensor. Also, if the potential sensor is soiled by toner floating in the device, a precise control cannot be performed, thereby causing the adhesion of the carrier.